1. Field of the Invention
The present invention relates to a combined cycle power plant having prime movers driven by gas turbines and steam obtained from the waste heat from the gas turbines, and to a method of operation of such a plant.
2. Description of the Related Art
In a combined cycle power plant system, it is required to cool a high temperature part of a gas turbine such as the turbine moving blade.
Increased efficiency of the operation of the combined cycle power plant can be achieved by increasing the gas turbine inlet temperature, but this temperature increase makes cooling of high temperature parts of the turbine more critical. Air cooling can consume a substantial part of the compressed air from the compressor driven by the turbine.
In view of the higher specific heat and thermal conductivity of steam, it has been proposed to use steam to cool the high temperature part of the gas turbine, as disclosed in JP-A-4-148035. In this proposed steam cooling gas turbine system, heat is exchanged between the exhaust gas of the gas turbine and feed water by a waste heat recovery boiler, and the generated steam is fed to the high temperature part of the gas turbine to cool it. At the time of start-up, when steam is not yet generated by the waste heat recovery boiler, or at the time of shut-down, the compressed air of the gas turbine compressor is fed to the high temperature part of the gas turbine to cool the high temperature part. This temporary use of compressed air is disadvantageous, however, because the cooling structure cannot be optimized for both steam and air as coolants. Moreover, a large amount of compressed air is required for cooling, reducing the amount available for combustion.
Start-up and shut-down of a combined cycle power plant may typically occur once per day each, and therefore a rapid and efficient start-up procedure is required.
Additionally JP-B-3-41644 describes the supply of steam from another source to drive the steam turbine of a combined cycle power plant during start-up. The gas turbine in this plant, however, is not steam-cooled.